In general, a semiconductor wafer is subjected to various thermal processes including a film deposition process, an etching process, an oxidation process, a diffusion process and a modification process to fabricate semiconductor integrated circuits on the semiconductor wafer. When a batch vertical thermal processing system is used for carrying out those thermal processes, semiconductor wafers are transferred from cassettes containing a plurality of semiconductor wafers, for example about twenty-five semiconductor wafers, to a wafer boat. The wafer boat is capable of holding a plurality of semiconductor wafers in the range of about thirty to about one hundred and fifty wafers depending on the size of the wafers. The wafer boat is loaded into a processing vessel from below the processing vessel, and then the processing vessel is sealed hermetically. Then, the semiconductor wafers are processed by a predetermined thermal process, controlling process conditions including flow rates of process gases, process pressure and process temperature.
The conventional processing vessel is surrounded by a heater for heating the wafers, and a heat-insulating layer of a heat-insulating material, such as alumina or silica. The heat-insulating layer serves to keep the processing vessel hot and to ensure the security of space around the processing vessel. Recently, it has been found that there is a possibility that metallic impurities, such as B, Fe and Cu, contained in the hot heat-insulating layer gradually permeate the processing vessel formed of quartz, enter the processing vessel and contaminate the wafers to some extent. A thermal processing system not provided with any heat-insulating layer has been developed to avoid such possibility. In view of improving throughput, it is desired to decrease the heat capacity of the entire processing furnace to enable heating a processing vessel at a high heating rate. From such a viewpoint, a thermal processing system not provided with any heat-insulating layer is desired.
A conventional thermal processing system not provided with any heat-insulating layer like the foregoing heat-insulating layer will be described by way of example with reference to FIG. 9. The thermal processing system 2 has a vertical processing vessel 8 of quartz having a double-tube structure consisting of an inner tube 4 and an outer tube 6. A quartz wafer boat 10 is held in a processing space S defined by the inner tube 4. The wafer boat 10 holds a plurality semiconductor wafers W at vertical intervals.
A cap 12 is arranged to open and close the lower opening of the processing vessel 8. A rotating shaft 16 is connected to the cap 12 through a magnetic fluid seal 14. A rotating table 18 is mounted on top of the rotating shaft 16, and a heat-insulating tube 20 is mounted on the rotating table 18. The wafer boat 10 is mounted on the heat-insulating tube 20. The cap 12 is connected to an arm 24 of a boar elevator 22 which is vertically movable. Thus, the cap 12 can be vertically moved together with the shaft 16 and the wafer boat 10. The wafer boat 10 is inserted through the lower opening of the processing vessel 8 into the processing vessel 8.
A stainless steel manifold 26 is joined to a lower end part of the processing vessel 8. A plurality of gas nozzles, namely, two gas nozzles 28a and 28B in the illustration, for supplying various process gases necessary for a thermal process (e.g., a film forming process) are extended through the manifold 26. Gas supply systems 30A and 30B are connected to the gas nozzles 28A and 28B, respectively. The gas supply systems 30A and 30B are provided with flow controllers 32A and 32B, such as mass-flow controllers, for controlling gas flow rates, respectively.
Process gases supplied through the gas nozzles 28A and 28B into the processing vessel 8 flow upward in the processing space S in the inner tube 4 in which the wafers are held, reverse at the upper end of the processing space S, flow downward through an annular space between the inner tube 4 and the outer tube 6, and are discharged from the processing vessel 8. A discharge port 34 is formed in the side wall of the manifold 26, and a not-shown vacuum pump for evacuating the processing vessel 8 is connected to the discharge port 34. A plurality of bar-shaped heaters 36 each extending vertically are arranged around the processing vessel 8 so as to surround the processing vessel 8. The heaters 36 heat the wafers W held in the processing vessel 8 at a predetermined temperature.
A cylindrical cooling jacket 38 not including any heat-insulating material is arranged outside the heaters 36. The cooling jacket 38 includes a cylindrical stainless steel inner shell 38A and a cylindrical stainless steel outer shell 38B, which define a space therebetween and are joined one another. A plurality of partition plates 40 are welded to the inner shell 38A and the outer shell 38B so as to form a meandering coolant passage 42 in the space between the shells 38A and 38B. When cooling water flows through the coolant passage 42, the cooling jacket 38 functions as a heat-insulating layer to keep a space around the cooling jacket 38 at a safe temperature.
As the pressure of cooling water flowing through the cooling jacket 38 is comparatively high, usually about 5 kg/cm2, the inner shell 38A and the outer shell 38B must have a comparatively high strength to withstand such a high pressure. Therefore, the shells 38A and 38B have a big thickness t on the order of 6 mm. Hence, the cooling jacket 38 is heavy, and consequently, a large, complicated support structure is necessary for supporting the cooling jacket 38. Since the partition plates 40 are welded at a space between the inner shell 38A and the outer shell 38B, the cooling jacket 38 is difficult to fabricate and needs a high manufacturing cost. If defective welding occurs, the cooling water will take a shortcut, and consequently, it is possible that irregular cooling occurs and wafers subjected to the thermal process are heated at widely different temperatures. If welding distortion occurs in or a weld bead is formed on the inner surface of the inner shell 38A facing the processing vessel 8 at a part to which the partition plate 40 is welded, the reflectance changes locally at that part and, consequently, the wafers are heated at different temperatures, respectively.